An image intensifier is an electronic device that produces a radiation pattern by electrically amplifying an image focused on the photosensitive surface of its photo cathode and presenting the amplified image on a fluorescent screen at the output. One widespread major application of image intensifiers is their use in night vision devices serving the military as well as other law enforcing agencies. Under very low illumination conditions, an image intensifier is capable of collecting and exploiting the low count of photons available in the environment and intensifying their image by factors of 50,000 to 100,000 providing good observation conditions in otherwise dark, barely illuminated places.
However, image-intensifying technologies are susceptible to harmful “glare” or “bloom” phenomena. Due to the fact that the entire field of view (hereinafter FOV) is intensified uniformly, a locally highly bright illuminated source can “blind” the system, causing possible permanent damage to the image intensifier. Such damage is caused by the light of the bright sources when intensified by a factor of approximately thousands times while being focused on the photo cathode.
To prevent the occurrence of such a potential damage, the gain of the Image Intensifier should be decreased or its exposure time (gating) should be shortened. Employing said reduction techniques; it is possible to protect the tube—albeit by doing so the brightness of the entire image evidently might be reduced—resulting in probable deterioration of the user's night surveillance performance. It is important to note that the action of providing protection from excessively bright sources by gating the number of collected incident photons that would be amplified, or by employing automatic brightness control that would reduces the power of intensification, affects uniformly the entire field of view and thus the overall performance deteriorates due to lowering the sensitivity and the contrast qualities of the system's image. Moreover, bright light sources create an additional interference due to the intensified processed image—namely the appearance of a halo around the displays of the bright areas.
The implementation of a micro mirrors array coupled to associated optics in order to control the intensity of the “processed intensified light” is described in U.S. Pat. No. 6,069,352. A control circuit determines whether a pixel's intensity is above or below a preset threshold level. In case it is above it, the corresponding elements of the micro mirror's array will deflect the incident light at that specific area, thereby eliminating saturation of the image intensifier from those pixels. Thus, the remainder of the image is unchanged and used for continued viewing. A continuous feedback loop monitors the intensity level of the pixels and actively controls the incident light governed by the micro mirror array.
The technology described in the above-cited US patent employs a Micro Mirrors Array (referred to as “MMA” in the terminology of that patent) for its operation. This device is also known in the field by the acronym DMD (Digital Micro Mirror Device) and appears in the literature also as MEMS (Micro Electro Mechanical System), as well as MOEMS (Micro Opto Electro Mechanical Systems) and of late also as “reflective spatial light modulator mirrors”—herein after: reflective MEMS. Such reflective MEMS can be commercially purchased off the shelf, for example from Texas Instruments, Inc.
In the application as described in the above-mentioned documented patent for using the reflective MEMS, there is a built in drawback: moving (shifting) one or more reflective MEMS mirror/s of the micro mirrors array, requires a given duration (time). For example, the elapsed time associated with the rotation of the mirror about a-hinge, a movement that calls for an appropriate response (reaction) time which is the time it takes until the unwanted glare is deflected from the image intensifier's input plane—unto which the reflective mirror was reflecting initially. Naturally, the mirror's movement, which lasts for some time, generates a “smearing” of the glare's light over substantial portion of Image Intensifier's input plane before deflected away from the input plane surface. This “smearing” is enhanced by the Image Intensifier itself, as long as the glare of the light keeps impinging on the image intensifier entrance plane, that is, until it will be totally deflected away from the entrance plane.
Another known practice is implemented by a technology known and recognized as “the flipping pixel”, which is bi-stable at 0°-180° or 0°-90°. It enables forming an array of mechanical nano—optical shutters that allow a fast control of whatever part of the area of a target that will receive the desired signal radiation and of the area that would not receive it and this subject to high spatial resolutions. A shutter array comprises a matrix of optical shutters positioned between a radiant source and a target. Every one of the shutters in the matrix is endowed by an “ON” state that allows light to pass through it and arrive at the target, and an “OFF” state that blocks the light from the source and prevents it from impinging on the target.
This referred to “other technology” is also known as “Micro Electro Mechanical Systems” (MEMS) and also as “Micro Opto Electro Mechanical System” (MOEMS) transmissive spatial light micros—hereinafter: transmissive MEMS. Such a technology is described, per instance, in the article “Bi-Stable Flat Panel Display Based on a 180° or Preferred 0°-90° Flipping Pixel” by Flixel Ltd. Company (at its site: WWW.flixel.com).